For many civil engineering projects the bearing capacity of existing soil on a building site must be increased in order to withstand the extreme weight of surface structures and also to minimize settling over a period of time. For example, in airport construction, proposed runway extensions often must extend into relatively soft or even marshy soil and such soil must be changed as to its bearing capacity before the runway surface can be installed. In other instances it often becomes necessary to form an impervious subsurface wall known as a cutoff wall in areas where the existing soil is relatively soft or marshy.
Various approaches to the aforesaid problem have been proposed by the prior art. In one previous approach, high permeability materials, such as columns of sand or artificial fabrics forming drain elements, were installed vertically at given intervals throughout the site to the entire depth of the soft soil in order to reduce moisture content of the soil which was responsible for producing settlement when structures were placed upon it. Subsequent to placing such vertical drains, a layer of sand and/or gravel was placed over the entire site. A surcharge in the form of a mound of soil was then placed over the area to provide added weight which tended to compress the soft soil. Water within the soft soil, which was now under pressure from the surcharge weight, gradually migrated to the adjacent vertical drains and was carried through them to the ground surface and eliminated. Accordingly, the moisture content of soft soil was slowly reduced. The surcharge load on the ground surface was maintained until the soft soil lost sufficient moisture content through the vertical drains and gained enough strength to provide the required bearing capacity and settlement characteristics. Use of such vertical drains in the construction industry started several decades ago. Some of the major examples of the use of such drain systems between 1954 through 1988 are described in the following articles:
(a) "Foundation and Fill Studies for the Metropolitan Oakland International Airport", Knappen-Tippetts-Abbett-McCarthy, Airport Consultant, New York, July 1954. PA1 (b) "Soft Clay Engineering", pp 650-669, Elsevier Scientific Publishing company, New York, 1981. PA1 (c) "Wicking Bay Mud", pp 53-55, Civil Engineering, American Society of Civil Engineering, December 1986. PA1 (d) "Osaka International Airport", pp 53-60, October, 1988, Korean Society of Civil Engineering, Vol. 36, No. 5. PA1 (e) "Hong Kong Replacement Airport", Journal of American Society of Civil Engineering, pp 87-146, Vol. 113, No. 2, February, 1987. PA1 (f) "Settlement, Consolidation and Use of Wick Drains", Craig Shields, Harding-Lawson Assoc., Symposium on the Geotechnical and Hydrological Properties of San Francisco Bay Mud, Lafayette, Calif., May 13, 1989.
One serious disadvantage with the aforesaid procedure was that the time required to obtain the desired effect was highly sensitive to the original soil condition, the surcharge weight, and the vertical drain intervals. Normally, use of the vertical drain procedure required two years or more to achieve the desired effects and, frequently, the predicted settlement of the soft soil under the surcharge load on the surface was in error. Once the vertical drain elements and the surcharge load were installed, no construction activities on the site were possible until the required settlement had taken place. Often, readjustment of the predicted settlement was required on the basis of survey results. A waiting period of two years or more for settlement to cease was expensive. Similarly, the installation of an impervious subsurface cutoff wall or barrier by the methods of the prior art was time-consuming and expensive.
Summarizing, the prior art method for increasing the bearing strength of soft and wet soil utilizing vertical drains in conjunction with surface surcharge materials inherently required a gradual reduction of the moisture content of soft soil, a process which required several years to complete due to the very low coefficient of permeability. Secondly, the gain of strength was primarily measured by the magnitude of measured settlement. The settlement rate was predicted on the basis of laboratory consolidation test results, which almost never agreed with the actual conditions at the site, and thus required a modification of the entire program. Lastly, after the protracted procedures of the prior method, the improved site still contained the original soft soil, although with a reduced moisture content and increased strength. However, the site was still subject to long-term settlement, and was also more susceptible to deformation during and after seismic activities.
A problem similar to that of increasing the bearing strength of soft soil is that of providing a subsurface cutoff wall in certain soft soil locations. In some instances where the soil mixture within a slotted wall construction is semi-fluid, a membrane or diaphragm may be lowered into the slotted wall to form an impervious barrier. As shown in U.S. Pat. No. 4,690,590, the movement of such a membrane downward through the soil may be increased by use of air bubbles supplied at the bottom of the membrane. The air bubbles serve to reduce the viscosity and thus the shearing stress in the boundary layer along the sides of the membrane. However, this patent provided no solution to the problem of creating a hardenable subsurface slotted or cutoff wall.